1. Field of the Invention
The present invention relates to a numerical control device that controls a machine tool and, more particularly, to a numerical control device that can invoke an external program.
2. Description of the Related Art
A machine tool manufacturer often stores an NC program specialized for a machine tool in a numerical control device, which controls the machine tool, and delivers the machine tool to an end user. The NC program specialized for the machine tool is contrived to make the best of functions and performance of the machine tool and is an intellectual property of the machine tool manufacturer.
In order to prevent the NC program specialized for the machine tool from being leaked to other machine tool manufacturers and end users, the numerical control device includes a function of preventing the NC program from being displayed on a display of the numerical control device, prohibiting editing of the NC program, and prohibiting an output of the NC program from the numerical control device to an external apparatus.
As a prior art, there has been disclosed a technique in which a numerical control device including an IC card reader can limit operation of the numerical control device based on ID information read out from the IC card reader and prohibit an output of an NC program (see Japanese Patent Application Laid-Open No. H4-184506).
A numerical control device connected to a network line includes a function of operating a machine tool while sequentially reading out an NC program stored in a host computer connected to the network line. This operation is called DNC operation.
As a prior art, there has been disclosed a technique in which a host computer transmits a plurality of NC data and schedule data created by the host computer to a control computer integrally provided in a machine tool and the control computer transmits the NC data to a numerical control device according to the schedule data to perform DNC operation (see Japanese Patent Application Laid-Open No. 2003-195930).
After shipping the machine tool to an end user, when the machine tool manufacturer provides the end user with an NC program specialized for the machine tool created by the machine tool manufacturer anew, the machine tool manufacturer has to visit the end user and store the NC program specialized for the machine tool created anew in the numerical control device.
In order to eliminate the labor and time, a DNC system is conceivable in which an NC program specialized for a machine tool is stored in a host computer, which is managed by a machine tool manufacturer, as an external program and a numerical control device connected to a network line invokes the NC program specialized for the machine tool from the host computer as the external program and executes DNC operation. However, this method has problem described below.    1. If the NC program is stolen from the network line, contents of the NC program are easily analyzed and illegally reused.    2. Since contents of the NC program running under the DNC operation is displayed on a display of the numerical control device, contents of the NC program are easily analyzed and illegally reused.